Cell Signaling basics. Local and Long-Distance Signaling Cells in a multicellular organism communicate by chemical messengers Animal and plant cells have.

Cell Signaling basics

Local and Long-Distance Signaling

• Cells in a multicellular organism communicate by chemical messengers

• Animal and plant cells have cell junctions that directly connect the cytoplasm of adjacent cells

• In local signaling, animal cells may communicate by direct contact, or cell-cell recognition

© 2011 Pearson Education, Inc.

Figure 11.4Plasma membranes

Gap junctionsbetween animal cells

Plasmodesmatabetween plant cells

(a) Cell junctions

(b) Cell-cell recognition

• In many other cases, animal cells communicate using local regulators, messenger molecules that travel only short distances

• In long-distance signaling, plants and animals use chemicals called hormones

• The ability of a cell to respond to a signal depends on whether or not it has a receptor specific to that signal


The Three Stages of Cell Signaling: A Preview

• Earl W. Sutherland discovered how the hormone epinephrine acts on cells

• Sutherland suggested that cells receiving signals went through three processes

– Reception– Transduction– Response


Animation: Overview of Cell Signaling

Figure 11.6-3

Plasma membrane

EXTRACELLULARFLUID

CYTOPLASM

Reception Transduction Response

Receptor

Signalingmolecule

Activationof cellularresponse

Relay molecules in a signal transductionpathway

321

Receptors in the Plasma Membrane

• Most water-soluble signal molecules bind to specific sites on receptor proteins that span the plasma membrane

• There are three main types of membrane receptors

– G protein-coupled receptors– Receptor tyrosine kinases– Ion channel receptors


Figure 11.7b

G protein-coupledreceptor

21

3 4

Plasmamembrane

G protein(inactive)

CYTOPLASM Enzyme

Activatedreceptor

Signalingmolecule

Inactiveenzyme

Activatedenzyme

Cellular response

GDPGTP

GDPGTP

GTP

P i

GDP

GDP

• A ligand-gated ion channel receptor acts as a gate when the receptor changes shape

• When a signal molecule binds as a ligand to the receptor, the gate allows specific ions, such as Na+ or Ca2+, through a channel in the receptor


Figure 11.7d

Signalingmolecule (ligand)

21 3

Gate closed Ions

Ligand-gatedion channel receptor

Plasmamembrane

Gate open

Cellularresponse

Gate closed

Intracellular Receptors

• Intracellular receptor proteins are found in the cytosol or nucleus of target cells

• Small or hydrophobic chemical messengers can readily cross the membrane and activate receptors

• Examples of hydrophobic messengers are the steroid and thyroid hormones of animals

• An activated hormone-receptor complex can act as a transcription factor, turning on specific genes


Figure 11.9-5Hormone(testosterone)

Receptorprotein

Plasmamembrane

EXTRACELLULARFLUID

Hormone-receptorcomplex

DNA

mRNA

NUCLEUS

CYTOPLASM

New protein

Signal Transduction Pathways

• The molecules that relay a signal from receptor to response are mostly proteins

• Like falling dominoes, the receptor activates another protein, which activates another, and so on, until the protein producing the response is activated

• At each step, the signal is transduced into a different form, usually a shape change in a protein


Protein Phosphorylation and Dephosphorylation

• In many pathways, the signal is transmitted by a cascade of protein phosphorylations

• Protein kinases transfer phosphates from ATP to protein, a process called phosphorylation


• Protein phosphatases remove the phosphates from proteins, a process called dephosphorylation

• This phosphorylation and dephosphorylation system acts as a molecular switch, turning activities on and off or up or down, as required


Receptor

Signaling molecule

Activated relaymolecule

Phosphorylation cascade

Inactiveprotein kinase

1 Activeprotein kinase

1

Activeprotein kinase

2

Activeprotein kinase

3


2


3

Inactiveprotein

Activeprotein

Cellularresponse

ATPADP

ATPADP

ATPADP

PP

PP

PP

P

P

P

P i

P i

P i

Figure 11.10

Figure 11.12

G protein

First messenger(signaling moleculesuch as epinephrine)

G protein-coupledreceptor

Adenylylcyclase

Second messenger

Cellular responses

Proteinkinase A

GTP

ATP

cAMP

Cell Signaling basics. Local and Long-Distance Signaling Cells in a multicellular organism communicate by chemical messengers Animal and plant cells have.

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